Cell-specific targeting for delivery of effector moieties such as diagnostic or therapeutic agents is a widely researched field and has led to the development of non-invasive diagnostic and/or therapeutic medical applications. In particular in the field of nuclear medicine procedures and treatments, which employ radioactive materials emitting electromagnetic radiations as γ-rays or photons or particle emitting radiation, selective localization of these radioactive materials in targeted cells or tissues is required to achieve either high signal intensity for visualization of specific tissues, assessing a disease and/or monitoring effects of therapeutic treatments, or high radiation dose, for delivering adequate doses of ionizing radiation to a specified diseased site, without the risk of radiation injury in other e.g. healthy tissues. It is thus of crucial interest to determine and assess cell-specific structures and in particular structures that are present in case of tumors (i.e. cancer) or inflammatory and autoimmune diseases, such as receptors, antigens, haptens and the like which can be specifically targeted by the respective biological vehicles.
The folate receptor (FR) has been identified as one of these structures. In normal tissues and organs FR-expression is highly restricted to only a few organs (e.g. kidney, lungs, choroids plexus, and placenta). Yet, the FR-alpha is frequently overexpressed on a wide variety of specific cell types, such as epithelial tumours (e.g. ovarian, cervical, endometrial, breast, colorectal, kidney, lung, nasopharyngeal), and the FR-beta is frequently overexpressed in leukaemia cells (approx. 70% of acute myelogenous leukaemia (AML) are FR-beta positive). Both may therefore be used as a valuable tumour marker for selective tumour-targeting (Elnakat and Ratnam, Adv. Drug Deliv. Rev. 2004; 56:1067-84). In addition it has recently been discovered that activated (but not resting) synovial macrophages in patients diagnosed with rheumatoid arthritis possess a functionally active FR-beta (Nakashima-Matsushita et al, Arthritis & Rheumatism, 1999, 42(8): 1609-16). Therefore activated macrophages can be selectively targeted with folate conjugates in arthritic joints, a capability that opens possibilities for the diagnosis and treatment of rheumatoid arthritis (Paulos et al, Adv. Drug Deliv. Rev. 2004; 56:1205-17).
Various folic acid derivatives and conjugates are known and have been (pre)clinically evaluated. In particular, folate radiopharmaceuticals have increasingly gained importance in the field of nuclear medicine and can be very useful for an improved diagnosis and evaluation of the effectiveness of therapy of cancer and inflammatory and autoimmune diseases, such as assessment and/or prediction of a treatment response and consequently improvement of radiation dosimetry. A typical visualization technique which is suitable for radioimaging is PET. PET uses isotopes with short half lives, which are either covalently linked to its carrier or via a chelating moiety. Suitable isotopes include for example 11C (ca. 20 min), 13N (ca. 10 min), 15O (ca. 2 min), and 18F (ca. 110 min) as covalently bound nuclides and for example 68Ga (ca. 68 min) which is usually linked by a chelating system.
Clearly a folate radiopharmaceutical having a covalently linked isotope would be of great interest. In particular a 18F-labeled folate radiopharmaceutical would be most suitable for PET Imaging because of its excellent imaging characteristics which would fulfil all of the above considerations. Compared with other suitable radionuclides (11C, 13N, 15O), 18F is very useful because of its long half-life of approximately 110 minutes and because it decays be emitting positrons having the lowest positron energy, which allows for the sharpest images with a high-resolution PET. Furthermore, the longer half-life of 18F also allows for syntheses that are more complex and satellite distribution to PET centers with no radiochemistry facilities.
Yet, the structure of folic acid does not lend itself to direct radiolabeling with 18F. Thus, to date, mainly chelate-based folate radiopharmaceuticals have been synthesized and successfully evaluated as diagnostic agents for imaging folate receptor-positive tumors. The most widely studied derivatives were labeled either with 111In and 99mTc for SPECT (Siegel et al., J. Nucl. Med. 2003, 44:700; Müller et al., J. Organomet. Chem. 2004, 689:4712) or with 68Ga for PET (Mathias et al., Nucl. Med. Biol. 2003, 30(7):725). In contrast, only very few folic acid derivatives have been reported in the literature which have been labelled with 18F (Bettio et al., J. Nucl. Med., 2006, 47(7), 1153; WO 2006/071754). Typically, an intermediate of choice was radiofluorinated to obtain an 18F-labelling intermediate, which is subsequently activated and purified in order to be subjected to coupling to a functional group within folic acid, such as the carboxylic acid group within the glutamate part of folic acid.
Clearly, such a multi-step radiosynthesis is time-consuming and in fact gave typically only low radiochemical yields of less than 5% (Bettio et al., J. Nucl. Med., 2006, 47(7), 1153).
Thus, there is still a great need for an efficient and versatile approach for preparing directly radiolabeled 18F-folates or derivatives thereof, which addresses one or more of the above discussed drawbacks.
Applicants have now found an efficient and versatile method of synthesis of new 18F-labeled folate radiopharmaceuticals overcoming the drawbacks of conventional labelling methods, wherein fluorine-18 is attached to a folic acid or derivative thereof through direct radiolabeling with 18[F]fluoride.
Thus the present method is a time-saving and convenient direct 18F-labelling method, wherein no prosthetic groups are necessary and suitable precursors which carry only amide bounded activated groups as moieties for direct 18F-labelling are easy accessible.
In addition the present method allows regioselective preparation and labelling of the α- or γ-isomer with no need for separation, which is known to be difficult and time-consuming.